Optical cells for housing samples during spectroscopic analyses are generally well known in the art. These devices facilitate in-line spectroscopic analysis of chemical samples, which is an important function in the chemical processing industry. In-line spectroscopic analysis enables the real-time determination of chemical content and concentration of chemical samples, both qualitatively and quantitatively, as the chemical is being generated and used in a chemical process.
Typically, a spectroscopic analyzer will employ an optical cell, an apparatus for introducing a chemical sample into the cell, a light source shining on the cell, and data collection and analysis instrumentation. Optical cells are available for use with various light sources, sample types, sample introduction methods, and collection and analysis modules, and may be used for either gaseous or liquid sample analysis. These cells are often used for off-line batch sample analysis, but in-line optical analysis cells with limited performance characteristics also exist. The optical cell itself is used to hold the sample adjacent to the light source during the analysis of the chemical sample. Known prior art optical cells typically use windows comprising a light transmitting material sealed to a pressure resistant housing.
These types of optical cells result in a number of disadvantages. Most significantly, sample fluids will often leak from the cells, especially at high-pressures, thereby soiling and/or damaging instruments, skewing test results, and/or necessitating time consuming cleanup procedures. This occurs because it is difficult to achieve a leak-free seal between metal cell-housings and the light-transmitting material. While in some cases, fused seals are used to provide leak-free operation, these applications are limited to batch measurement, result in undesirable dead volume, and/or are not readily reusable due to the time and expense required for cleaning. More generally, such cells comprise numerous parts and are unnecessarily complex. Such arrangements generally include at least two crystalline windows, two-piece adjustable housings, and two-piece fused cells. This abundance of parts increases the possibility of leaking, especially at high pressures.
Accordingly, it has been suggested to use an optical cell formed from a monolithic crystal, to which the sample inlet and outlet tubes are sealed, such as that disclosed in U.S. Pat. No. 5,949,536 to Mark, the specification of which is hereby incorporated by reference. This optical cell solves many of the disadvantages inherent in previous optical cell designs, including the assembly of separate parts and the undesirable leakage that often results.
The use of these optical cells, however, can be limited in terms of the applications in which they can be employed. An improved optical cell, which allows one to look into the cell, rather than being limited to spectra transmitted through the cell, would allow one to measure the spectra of samples within the cell using reflection methods, or Raman spectroscopy, while being able to see a clear image of the sample housed in the cell.
What is desired, therefore, is an optical cell for spectroscopic analyzers that performs well at high pressures. What is further desired is an optical cell that is not limited to spectroscopic applications that collect spectra through the cell. What is also desired is an optical cell that is inexpensive, is easy to assemble and clean, and minimizes undesirable dead zone.